This invention relates to a jack for lifting heavy equipment such as bulldozers, haul trucks, scrapers, and the like, where high lift of large tonnages is required. The invention also relates to a truck for positioning and transporting the jack.
Jacks presently available for high lift of large tonnages have a number of disadvantages. Jacks having a combination of large tonnage capacity and long stroke usually comprise telescopic cylinders stood vertically under the equipment to be lifted. Both double acting and single acting jacks are known. Single acting jacks require large external springs to retract the cylinder making the machine bulky while not being 100% reliable in retraction. Both the double acting and single acting units have a major disadvantage when used for lifting heavy loads. Depending on seal drag, occasionally the second stage of a two-stage jack extends first, lifting the machine off the ground, until the increased pressure overcomes the breakaway friction on the first stage seals. Once this occurs the pressure to move the first stage is much lower than on the second stage. This causes the second stage to drop as the hydraulic fluid behind the second stage piston is forced out and under the first stage piston. The first stage is extended by this fluid but not by the same distance as the retraction of the second stage. The result is usually a sudden drop of the load several inches. This is disconcerting to the operator of the jack and can be dangerous. A similar situation can occur when lowering the load.
Other shortcomings of existing heavy equipment jacks concern the means for transporting and positioning the jacks. When wheels are fitted, a mechanism is required to ensure that the wheels do not take the load when the jack extends in lifting heavy equipment. One design allows the jack to hinge on a spring loaded mechanism. In another design, movement of the jack into a vertical position is required to lift the wheels clear of the floor.
Other jacks have either a long frame (handle) which can be used to place the jack in position, or require the operator to go under the equipment to position the jack. Jacks having a long frame fitted usually have it positioned only about 600 mm above floor level so that the frame does not foul the equipment. Due to the weight of the jack, a great deal of effort is required to move it, sometimes more than one person being needed. Use of this amount of effort on a handle so close to the floor can cause back injuries.